1. Field of the Invention
The present invention relates to a sprueless disc mold and a disc molding method for injection-molding a recording medium disc substrate such as a video disc or a compact disc (as abbreviated to CD) having an opening at its center.
2. Related Art
Generally speaking, the disc substrate is manufactured by an injection-molding process using a disc mold. As shown in FIG. 11, this molding process is performed by charging a cavity 56, which is formed between a mirror plate 50 of a stationary molding member and a stamper 54 mounted on a mirror plate 52 of a movable molding member, with a molten resin injected from the (not-shown) nozzle of an injector through a sprue hole 58.
The stamper 54 is used to transfer the helical or concentric pits, which are formed with surface corrugations having a width of about 0.5 microns, as recording information data to the disc. This stamper 54 is fixed by having its inner circumferential edge and outer circumferential edge held on the mirror plate 52 of the movable molding member by the flanged portion 60a of a stamper holder 60.
After the molten resin charged in the mold has been solidified to a predetermined hardness, the disc substrate has its center opening 66 (as shown in FIG. 12) punched out by a center punch 64 which is enabled to slide therein by using a sleeve 62 as a guide member.
Here will be described a disc substrate 1 to be molded by the mold described above.
The disc substrate 1 has its shape schematically shown in a top plan view and in a section in FIGS. 12 and 13. This disc is formed with at its center with the opening and around this opening 66 with an annular groove 68 to be formed as a recess by the flanged portion of the stamper holder, and is divided across the annular groove 68 into a clamp area 70 at the inner side and a signal area 72 at the outer side. To this signal area 72, there are transferred by the stamper 54 the pits which are formed in the helical or concentric shape.
Moreover, the disc substrate 1 is thus molded by an injection-molding apparatus and is then stacked on a stacker. In order that, in this case, the disc substrate may be prevented from contacting with and being damaged by another while promoting the heat dissipation from the substrates, each of these substrates is formed with an annular stack rib 80 on its back providing the recording information face. This stack rib 80 is formed either in a radially outward or inward offset from the annular groove 68 arranged to face it on the surface or in the same position but with a larger width than that of the annular groove 68.
In this molding process of the disc substrate, the step of punching out the center opening is included to elongate the time period for the molding cycle, and powdery chip is produced by cutting the disc internal diameter through the punching step so that it is caught by the electrified disc substrate or by the stamper in the mold. As a result, there arises a problem that the chip is mixed into the disc substrate at a next shot, to deteriorate the production yield of the molding.
Thus, there is raised by the makers a movement that the molding cycle is to be shortened by omitting the step of punching out the center opening, which has been required in the prior art. The sprueless mold is noted as the disc mold for satisfying the movement. By adopting the sprueless mold using a hot runner, according to the recent trial, the disc substrate is manufactured not only by solving the defect of the prior art but also by eliminating the sprue and the runner for the passages of the molten resin to minimize the loss of the material.
The mold of this kind is disclosed in U.S. Pat. No. 4,394,117, 4,391,579, 4,340,353 or 4,405,540 or Japanese Patent Laid-Open No. 212757/1993, for example.
The mold, as disclosed in U.S. Pat. No. 4,394,117 (corresponding to Japanese Patent Publication No. 23972/1985) to be exemplified herein, is used to manufacture a recording disc having a center opening by using an injection-molding apparatus and is equipped with a valve assembly for controlling the flow of a molten resin to be charged in the mold cavity.
This valve assembly is exemplified by a sprue bush 100 having a hot runner structure, as shown in FIG. 14. This sprue bush 100 is constructed of: a stem 106 having a heating coil 102 mounted around a resin passage 103 and a conical dispersion head 104 mounted on its lower portion; and a sleeve valve 108 fitted slidably on the stem 106. The sleeve valve 108 is so urged to a protruded position, i.e., a closed position by spring means 114 such as a dish spring as to bring a conical face 110 of the dispersion head 104 and an annular slope 112 on the lower edge of the sleeve valve 108 into abutment against each other, but normally prevents the molten material from flowing into a cavity 116.
When the molten material is injected under pressure through the sprue bush 100, it axially flows to the dispersion head 104 and passes through a plurality of radially extending grooves until it reaches the gate reservoir. If the pressure in the sprue bush exceeds a predetermined level, the sleeve valve 108 rises in the direction of arrow A in response to that pressure until it is automatically retracted to an open position. As a result, the gate is opened by establishing a gap between the slope 112 on the lower edge of the sleeve valve and the conical face 110 of the dispersion head, to charge the cavity with the molten material.
After this, if the pressure to be applied to the molten material is lowered to a level lower than the valve retracting pressure, the force to be applied to the sleeve valve 108 by the spring 114 overcomes the force acting upon the pressure face of the valve, to push the sleeve valve 108 back to the closed position.
Since, in this case, the material in the vicinity of the sleeve valve is pushed back to the bore of the sprue bush by the inner slope at the leading end of the valve, the material in the cavity can be prevented from being locally disturbed, and the pressure fluctuation of the molten material in the sprue bush can also be prevented from being transmitted to the inside of the cavity by closing the sleeve valve.
The sleeve valve 108 thus constructed has an external diameter corresponding to the internal diameter of the center opening for forming the information disc, and the disc is molded with the center opening on the basis of the contour of the valve as a result that the valve moves to the closed position before the molten material solidifies. This makes it unnecessary to punch out that opening.
In the valve assembly of this type, however, the disc opening is formed such that the molten resin is separated by the leading end portion of the sleeve valve, so that the shape of the opening portion and the finish of the surface when the resin solidifies are not always satisfactory without neither any step nor any bur. Since, moreover, the structure is made such that the resin retains its passage opening by its own flow, it is limited by the fact that the control for starting the separation of the center opening before the end of injection cannot be achieved.
On the other hand, the disclosure of U.S. Pat. No. 4,340,353 (corresponding to Japanese Patent Publication No. 23971/1985) seems to have any substantial difference from the center opening forming concept of the prior art in that the valve is moved to separate the center opening.
In addition, another sprueless mold for the disc having the hot sprue bush is disclosed in U.S. Pat. No. 4,439,132 (corresponding to Japanese Patent Publication No. 13808/1988) or 4,412,805, for example.
The mold of this kind is equipped, as shown in FIG. 15, with a sprue bush 122 for forming an annular gate 120 in a slightly recess plane on the disc face, as spaced from the center opening of the disc. This sprue bush 122 is given a hot runner structure in which a conical dispersion head 130 is fixedly arranged in abutment against the bottom of a step 128 having a heat cartridge 124 therein and a heating coil 126 arranged around its upper portion. A resin passage 134 is formed in a conical face 132 between the stem 128 and the dispersion head 130.
In this hot runner, as shown in FIG. 16, the dispersion head 130 is formed with a plurality of spacer steps 136, which are circumferentially arranged to inject the molten resin radially outward, so that the molten resin flowing from the center hole 138 of the stem is divided into homogeneous annular flows to pass between the conical faces to the cavity. Moreover, the resin material thus annularly dispersed through those spacer steps 136 is accelerated while flowing toward the cavity so that it does not solidify with an excellent flow to the gate.
In this case, however, the sprue bush 122 and the dispersion head 130 are fixed to each other so that the gate is always opened. As a result, the gate mark is left on the face of the molded disc. If this gate mark rises from the disc face, a post-treatment for removing it may become necessary.